A synchro is a rotary transducer that converts angular displacement into an AC voltage or an AC voltage into angular displacement. The common form of synchro is a relatively simple device having multi-turn 120.degree. space-phased windings on its stator and a single multi-turn winding on its rotor. A typical, or two element, synchro system utilizes two such synchros with the stators of each being connected in parallel. In such a two element synchro system, an AC carrier or reference voltage, typically 60 or 400 Hz, is applied to the transmitter synchro or control transmitter rotor and the rotor position establishes a flux vector in the stator, generating particular voltages in each leg of the stator winding. These voltages are transmitted to the stator of the receiver synchro or control transformer where a similar vector flux pattern is reproduced. The rotor of the control transformer then aligns itself in the flux field.
A resolver is a type of synchro and is often called a "synchro resolver". A major difference between a synchro and a synchro resolver, or simply a resolver, is that the multi-turn stator and rotor windings of the resolver are displaced electrically 90.degree. to each other instead of 120.degree. as in the case of the synchro. The common form of resolver has a single rotor winding and two stator windings, each with many turns per pole. When the rotor of the resolver has an AC carrier voltage V.sub.in applied thereto, the voltage across one of the stators becomes: V.sub.1 =V.sub.in K Sin .theta., and the voltage across the other stator becomes: V.sub.2 =V.sub.in K Cos .theta., where K is a constant.
Thus, like a synchro, a resolver is a device that is used for the transmission, reception or conversion of angular data. A resolver is also a type of rotary position indicating transducer that produces two voltage outputs, one, V.sub.1 varying as a sine function; and the other, V.sub.2, varying as the cosine function, according to the angular position of a rotor with respect to a stator.
Another form of resolver has two stator and two rotor windings, each with many turns per pole, and the reference or carrier excitation voltage may be applied to any of the windings. If one of the stator windings are used as the input and excited, the unused stator winding is shorted. The voltages across the rotor windings are monitored as the output voltages and would be given as above for V.sub.1 and V.sub.2. Alternately, one of the rotor windings can be used as the input with the two stator windings being used as the outputs.
It may be explained here that synchro resolvers, or resolvers, are of two main types, brushless and non-brushless. A non-brushless resolver is one whose excitation winding is excited by means of a sliding contact with an AC source. A brushless resolver is a resolver in which the excitation winding is excited without the use of a sliding contact. One type of brushless resolver is a transformer brushless resolver. With a transformer brushless resolver, for example, the rotor winding is excited through an inductive coupling to an AC source.
Synchro resolvers or resolvers have traditionally been made with wound rotor and wound stator windings. Manufacture of such devices is labor intensive, and, therefore, expensive. Moreover, resolvers and, in particular brushless resolvers, are relatively heavy and have a large axial dimension. A need and demand has arisen for a resolver, and in particular for a transformer brushless resolver, that is inexpensive, light in weight and that has a relatively short axial dimension.
It would be desireable to provide a resolver that is inexpensive to manufacture, light in weight, and small in axial dimension. In order to achieve this objective, it would also be desireable to provide a resolver that employed printed, rather than wound, rotor and stator windings.
Various types of printed circuit rotary position indicating transducers are known, including both capacitive and inductive types. Though they perform some of the same functions as resolvers, neither type of device is, in fact, a resolver.
With capacitive type rotary position indicating transducers, a rotor having conducting capacitive plates affixed thereto, rotates with respect to a stator having associated conducting capacitive plates. The capacitance across the plates varies with their relative angular position. One disadvantage of capacitive rotary position indicating transducers resides in the fact that they are susceptible to shock and vibration. Shock and vibration may vary the dimensions of the air gap between the plates and, this, in turn, will vary the capacitance. A capacitive type rotary position indicating transducer is shown in U.S. Pat. No. 4,092,579-Weit. Though the transducer is referred to in the patent as a resolver, the device is not technically a type of resolver because it does not employ multi-turn windings.
Inductively coupled rotary position indicating transducers are also known. One such transducer is known as an Inductosyn position transducer and is described in a brochure entitled "Inductosyn Position Transducers" published by Farrand Industries, Inc. Such transducers employ printed circuit undulating conductive patterns, with a half turn per pole. The patterns have many hairpin turns that are situated along the flat surface of both a rotor disc and a stator disc. The length of one complete cycle of the hairpin pattern is called the pitch. One of the discs has a single winding pattern and the other has a first and a second winding pattern thereon with the second being displaced a predetermined distance from the first. Alternating current applied to the single winding on one disc as an excitation signal results in two output signals from the two windings on the second disc. The two output signals have amplitudes which vary as sine and cosine functions based on the relative position in the pitch cycle. There is a unique pair of sine and cosine output amplitudes for every position within one cycle of the pitch.
The Inductosyn, in actual operation, is similar to a resolver in that a sine and cosine function are derived. The device differs from a resolver, however, because it does not employ multi-turn windings. Because the Inductosyn does not employ multi-turn windings it suffers from several drawbacks. First, these devices are capable of directly sensing only very small angular displacements. If larger displacements are to be sensed, the devices must be used in conjunction with logic circuitry to unambiguously determine angular position greater than a few degrees of arc. Because of the need for such logic circuitry, the devices are expensive, costing as much as several thousand dollars per unit. Secondly, the transmission ratio, i.e., the output voltage divided by the input voltage, of such devices is very low. With typical wound rotor/wound stator resolvers the transmission ratio is much higher. Because of their low transmission ratio, the output voltage of such transducers must be amplified and the use of an amplifier contributes to the increased cost of such devices.